High frequency oscillator



Oct, 26, 1937.

A. H. TURNER HIGH FREQUENCY OSCILLATOR Filed July 31, 1935 2Sheets-Sheet l A. H. TURNER HIGH FREQUENCY OSCILLATOR Oct. 26, 1937.

Filed July 31, 1955 I 2 Sheets-Sheet .L usual/m may K9 3 Patented Oct.26, 1937 UNITED STATES PATENT OFFICE to Radio Corporation of America,

tion of Delaware Application July 31, 1935, Serial No. 33,955

9 Claims.

My invention relates to oscillators. Specifically, my invention is animproved high frequency oscillator having substantially constantfrequency over a wide range of plate and heater potentials. Y

one object of my invention is an improved ultra high frequencyoscillator. v 3 Another object is an oscillator circuit havingsubstantial constant frequency over a wide'range of plate voltage-andheater potentials. I

--Another object is to provide a high frequency oscillator whose voltageoutput is relatively constant within the tuning frequency range.

A still further'object is to combine, in a single oscillator, thecharacteristics of constant frequency and a constant coupling outputcircuit.

7 Other objects will appear from the following specification and claims.

Figure I is a schematic diagram of a conventional oscillator,

Figure II is a diagram of an oscillator employing a tracking condenser,

Figure III is a diagram of an oscillator embodying one form of myinvention,

Figure IV is a modification of Figure III,

Figure V is a graph showing the variation in output frequency withchanges in plate voltage,

Figure VI is a graph showing the variation in output frequency withchanges in heater voltage,

Figure VII is a graph showing the variation of 'grid'an'd plate currentwith frequency, and

Figure VIII is a graph showing the variation of frequency with varyingplate voltage for difierent values of grid leak resistance.

The circuit illustrated in Figure I is a vacuum tube oscillatoremploying a tuned grid circuit and tickler' feedback. This circuitarrangement is well known to those skilled in the art. A circuit of thistype has objectionable characteristics. 40 The oscillatory voltagesacross the tuned circuit will be substantially reduced as thefrequency'is decreased. If used in a heterodyne receiver, the oscillatorwill not track with tuned'preselector circuits unless special variablecondensers are employed. The greatest difiiculty is experienced at highfrequencies where the frequency cannot be stabilized to any substantialdegree. A further undesired variation in oscillator frequency takesplace with changing plate and heater potentials. This latter effectbecomes very serious at high frequencies/ I am aware of numerousarrangements for overcoming these dilficulties. Some of thesearrangements are satisfactory at broadcast frequencies but are notapplicable at ultra high frequencies of the order of 30 megacyclesupward. Some of the theory of oscillators applicable to low frequenciesis far from rigorous at ultra high frequencies. My present inventionminimizes and compensates the deleterious effects usually experienced inhigh frequenc'y'oscillator circuits. A 1 3 In Figure II is illustrated amodified form of oscillator similar to Figure I. In Figure II, a gridinductance l is connected through grid 'condenser 3 to grid 5 ofthermionic tube 1. The 10 lower end of the grid inductance is connectedto plate inductance 9 which is connected to plate II. The junction ofthe grid and plate inductances is coupled through tracking'condenser l3to the cathode l5 which is grounded. A variable tuning capacity I! isconnected from the upper terminal of the grid inductance to the cathodeand ground. A grid leak resistance I9 is connected between grid andcathode. The B battery 2! f has its negative terminal grounded and itspositive terminal connects to the upper end of the plate inductancethrough the radio frequency-choke 23. The B battery may be by-passed bya suitable capacity 2 5. The cathode I5 is heated by filament 27.Inductances l and 9 are preferably mutually coupled.

Although the circuit of Figure II maybe ade justed by properly choosingthe value of l3 to track with tuned circuits in a superheterodynereceiver or the like, it still has'objectionable char 0 acteristics atfrequencies'of theorder of 30,megacycles and upward. In Figure V thecurve A represents the eifect of varying the plate voltagejo n thefrequency output of the circuit of Figure II. 5 It is customary tosupply the plate voltage from a rectified filtered source of powerderived from a commercial sixty cycle volt. supply. Such sources havebeen known to Vary as much as twenty-five percent in the case of poorlyregulated lines and as much .as thirty percent in different sections ofthe country. Such variation wouldhave a serious eifecton the oscillatorof asuperheterodyne. v

A similar variation in a tuned grid [circuit oscillator, although in theopposite direction, is I found in Figure VI, curve C. In this curve theeffect of varying heater voltage in terms of frequency change is shown.The curves E, andF, of Figure VII, illustrate the variation of grid '50and plate currents with frequency in the case of a tuned grid circuitoscillator. Since the effective output of oscillatory current isproportional to the grid current, the large variation of output in theoscillatory current of a. tuned grid circuit oscillator is objectionablefrom a constant current standpoint. I

. The foregoing examples of variation in the frequency output of a tunedgrid circuit oscillator apply especially when the 1 frequency range isfrom about 30 megacycles and upward. Although the curves are based onobservationsof the characteristic of an RCA type 955-Acorn type tube,

' it should be-understood that similar frequency .variations are presentin other types of tubes connected to similar circuits.

Many variable factors make it diillcult to mathematically treat thetubesand'associate circuits to derive equations which explain thevariations. Among the important variable factors at ultra highfrequencies is the recently recognized variation caused by changes ofspace charge and electron velocity within the tube with changing. plateand'filament potentials. This variation ap:v

pears to alter the efiective'interelectrode capacitieswithin the tube. VSomevariable factors-may be minimizedand otherscompensated.

One important efiect at high frequencies, is the variation'in theeffective grid, cathode and/or grid plate capacity with variations inplate circuit' impedance. primarily- ;due to-a decrease in the-effectivegrid to cathode'capacity, If the oscillatory circuit is 6 inserted inthe plate circuit, variations in effective capacity between grid andcathode will have little-eifect, because the tuned plate circuitprimarily determines the frequency.

r Figure III is a diagram of a tuned plate circuit oscillator: Aninductance3l is connected through grid condenser 33 to grid 35 of tube31. The lower end of. 3! is connected to inductance 39. The lowerterminal of39 is connected to plate 4|. The

junction'of 3land39'is connected through track-1 ing capacity to cathode45 which is grounded. A variable tuning capacity 41 isconnected'between; ground and the lower terminal of inductance 39.; Agrid leak 49 is connected between fandthe tuning' of theplate'circuit ofthe latter. -W'hile 'a comparison of, diagrams indicatesa similarity, acomparison of graphs, Figures V, VI

and. VII; discloses substantialjdifierences. ,In' Fig! N we V the curveBshows a substantially constant frequency withvarying plate voltage.A'comparison of A1 and B indicates a very'great. im-

provement. In like manner curve D of Figure VI shows that fIOIIL'T voltsto 5.3 volts or heater potential the frequency.is practically unchanged.Curve C" shows a substantial variation of iregquency with decreasingheater voltages beginning, at.6.3 volts. The" curves shown in Figure VIIindicate that the radio frequency output which accompanies curve G ismoreconstant than thatindicated by curve E; Because of the shape ofcurve G the variations in output with ire- "quency may be compensated byvarious types of compound coupling linking the oscillator output and thework circuit. Such compound coupling keeps the applied voltage constantwith changes in oscillator frequency.

. 'While the principal gain in frequency stability illustrated bycurves'B and D of Figures V and The increase infrequency is VI,respectively,is due place of grid circuit tuning; the results are notentirely due to this. change. bility indicated by curve B is due tocompensating 'means I employ. The circuits of the several figures, whenemploying a grid leak resistance of the order of one hundred thousandohms, show an increase in frequency Withincreasing plate potentials. Ihave observed the efiect of lowering the grid leak resistance is tocause a change in the frequency characteristic with changing platevoltage which over a portion of the range is opposite to the usualfrequency increasing tendency.

The result of such observation is found in Figure 7 VIII. The curves Jand K with 100,000 ohm grid leak resistances show a rising frequencycharac-.

teristic with increased plate voltages. The curves L and-M show a risingfrequency characteristic at low voltages and a reversal of slope forhigher voltages. By a suitable choice of operating volt age and gridresistance, the" ordinary rising fre- Part of the sta-" to plate circuittuning in 7 quency characteristic withincreasing plate volti- I age,maybecompensatedaln ciu'ves L-anddMa valueof grid leak resistance is.employed which approaches the optimum value for a constant frequencyoutput with increasing; plate voltages;

Figure IV is a diagram showing a modification or Figure-III} Thegridcircuit inductance is connected through gridcondenser 63' and leak 65 togrid 61' of tube. The lower terminal so L of 6| is connected to cathode1i and ground. The I plate'cir'cuit inductance 13isconnected'between theplate 15 and the positive-terminal of B battery l'l. ;The B battery has;its negative terminal grounded. The variable tuning capacity 8I- isconnected between ground andthe junction of inductance 13* and plate 15.The tracking condenser 83 is connected "between theylowerlterminal of 6|and the upper, terminal'of 13;. The

condenser 83 also acts as a by-pass for battery I1.

The essential'difierencebetweenthe circuits shown in Figures III and IVis that in the latter the.- grid leak resistance is serially connectedtothe grid circuit and has a smaller loading efiectin-thiS position. Thelesser loading effect of thegrid resistanceis an advantage in a numberof circuit arrangements t Y Although my invention may be applied to anyoscillator, it is primarily designed for a high frequency oscillatorandis particularly suited. to a superheterodyne receiver.

The tracking condensers i3; and43 of Figures II and III are suit ablefor aligning the oscillator with respect to the preselector circuits.The combination of the mutual inductive coupling and the capacity ofthese condensers increases the coupling between V the grid and platecircuits at the lower frequencies, and tendsto-keepthe' outputoscillations constant in amplitude throughout the frequency range. Inmany cases; I find the tracking condenser capacity must 'be large withrespect'to 1 the tuning capacity. This large ratiois not the optimumfor'impressing' a constant voltage on the 'workcircuit. In such cases, Iemploy, a

compound coupling between the oscillator and first detector or workcircuit. The compound coupling keeps the impressed voltageonthefirst,

detector substantially constant as the oscillator is: tuned throughoutthe frequency range.

By way offexample Ihave employed the following constants in the .circuitof'Figure'III: Grid circuit inductance two turns number I! enameledcopper wire onv 1 diameter form. Plate circuit inductance four turnsnumber Is enameled copper wire on lf'e diameter form.

2,096,829 Both the grid and plate circuit inductances are a continuouswinding of six turns spaced about one-eighth of an inch on a commonform. The tuning condenser is variable from five micromicrofarads totwenty micromicrofarads. The common condenser is eight hundredmicromicrofarads. The grid condenser is thirty-five micromicrofarads andthe grid leak about 10,000 ohms. An Acorn tube RCA 955, is preferred. Aplate voltage of volts may be used. The rated heater voltage of 6.3volts is employed.

The circuits described cover arrangements of my invention which givesubstantially constant high frequency outputs with varying plate andheater potentials. The amplitude of the output current is substantiallyconstant over the tuning range, and the voltage applied to a workcircuit may be made uniform by employing the above mentioned compoundcoupling. The circuit diagrams, curves, and constants are given merelyby way of example. I do not intend to limit my invention to the precisearrangements shown because other obvious arrangements within the scopeof my invention will occur to those skilled in the art.

I claim:

1. In an ultra high frequency oscillator, a thermionic tube having grid,cathode, and plate electrodes with relatively low interelectrodecapacities; means for moving electrons from cathode to plate whereby theeffective interelectrode capacities are varied; means for tuning theplate circuit throughout a range of ultra high frequencies; aninductance serially included in the grid circuit; means connected insaid grid circuit hav ing an impedance which establishes an increas ing,then a decreasing frequency characteristic in said oscillator withincreasing plate voltage; and means mutually coupling said grid andplate circuit, whereby the effect of said moving electrons on thefrequency characteristic of said oscillator is substantially reduced.

2. In an ultra high frequency oscillator, a thermionic tube having grid,cathode, and plate electrodes with relatively low in'terelectrodecapacities; -means for moving electrons from cathode to plate wherebythe interelectrode capacities are apparently varied; means for tuningthe plate circuit throughout a range of ultra high frequencies; aninductance, and grid condenser serially connected between grid andcathode; means connected in said grid circuit having an impedance whichestablishes an increasing then a decreasing frequency characteristic insaid oscillator with increasing plate voltage; and means mutuallycoupling said grid and plate circuits, whereby the effect of said movingelectrons on the frequency characteristic of said oscillator issubstantially reduced.

3. In an ultra high frequency oscillator, a thermionic tube having grid,cathode, and plate electrodes with relatively low interelectrodecapacities; means for moving electrons from cathode to plate whereby theeffective interelectrode capacities are varied; means for tuning theplate circuit throughout a range of ultra high frequencies; aninductance, and grid condenser serially connected between grid andcathode; means mutually coupling said grid and plate circuits, wherebythe effect of the apparent changes in inter-electrode capacities on thefrequency of characteristic is minimized; and a grid leak resistanceconnected between grid and cathode having a value which gives saidoscillator a frequency characteristic which increases then decreaseswith increasing plate voltage for compensating the frequency variationcaused by the apparent change in interelectrode capacity.

4. In an ultra high frequency oscillator, a thermionic tube having grid,cathode, and plate electrodes with relatively low interelectrodecapacities; means for moving electrons from said cathode to platewhereby the effective interelectrode capacities are varied by saidmoving electrons; means for tuning the plate circuit throughout a rangeof frequencies; an inductance and grid condenser serially connectedbetween grid and cathode; a grid resistance effectively connectedbetween grid and cathode and having an ohm-i0 resistance whichestablishes a frequency characteristic which increases then decreaseswith increasing plate voltage; and means mutually coupling said grid andplate circuits, whereby the effect of said moving electrons on theinterelectrode capacities is minimized as to frequency changes in theoscillator.

5. In a device of the character of claim 1, a fixed capacity common tothe grid and plate circuits.

6. In a device of the character of claim 1-, a fixed capacity seriallyincluded in the tuned plate circuit and common to the grid and platecircuits. 7

7. The method of generating ultra high frequency currents of constantfrequency in a thermionic tube having control grid, cathode, and plateelectrodes; a resonant output circuit; a grid circuit coupled to saidoutput circuit by means of mutual capacitance and mutual inductance; anda source of power subject to variations whereby the movement ofelectrons in said tube is afiected, which comprises generating apotential difference across said resonant output circuit, inducing insaid grid circuit from said output circuit by means'of said mutualcapacitance and said mutual inductance a voltage in proper phase tosustain said ultra high frequency currents, and limiting the impedanceof said grid circuit to a value which establishes a frequencycharacteristic in said oscillator which increases then decreases withincreasing plate voltage and is substantially uninfluencedby apparentcapacity changes caused by said movement of electrons, whereby constantultra high frequency oscillations are maintained regardless of thevariations in said power source.

8. The method of generating constant ultra high frequency oscillationsin a thermionic tube having input and output circuits and avoiding theeffect of apparent capacity changes in said tube on said frequency whichcomprises resonating the output circuit of said tube to the desiredfrequency, generating voltages across said output circuit, feeding fromsaid output circuit voltages to said input circuit having capacitive andinductive components to sustain said resonant output voltages, andsubstantially limiting the efiects of said apparent capacity changes bylimiting the impedance of said input circuit to a value which will havean increasing and decreasing efiect on frequency over a range of platepotentials whereby a constant ultra high frequency oscillation isgenerated regardless of said apparent capacity changes.

9. The method of generating constant ultra high frequency oscillationsin a thermionic tube having input and output circuits coupled by mutualinductance and mutual capacity, and avoiding frequency change caused byvariations in the power supplied to said tube and changes in cacomprisesresonatingthe output circuit of 7 said tube, generatingvoltages in 'saidoutput circuit by feeding back voltages through said mutual 7 couplingto said input circuit, amplifying input voltages in saidoutput circuit,and adjusting the impedance of said input circuit to a value whichestablishes an increasing and decreasing frequency characteristic insaid'oscillator with increasing plate voltage and'which reflectssubstantially no reactive component on said resonant output circuitregardless of said power and 5 capacity variations.

ALFRED H1

